GRB 250321A

The Fermi GBM team reports the detection of a likely LONG GRB

At 00:42:39 UT on 21 Mar 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250321A (trigger 764210564.016716 / 250321030).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 299.0, Dec = 17.1 (J2000 degrees, equivalent to J2000 19h 56m, 17d 06'), with a statistical uncertainty of 6.3 degrees.

The angle from the Fermi LAT boresight is 44.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250321030/quicklook/glg_skymap_all_bn250321030.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250321030/quicklook/glg_healpix_all_bn250321030.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250321030/quicklook/glg_lc_medres34_bn250321030.gif

R. Gupta (NASA GSFC), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), D. M. Palmer (LANL) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 00:42:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250321A (trigger=1297508).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 295.077, +21.041 which is 
   RA(J2000) = 19h 40m 19s
   Dec(J2000) = +21d 02' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Only the first 8 seconds of the BAT light
curve after the trigger is available due to a telemetry gap.
This data shows a complex structure with a duration of
at least 10 sec.  The peak count rate was ~2500 counts/sec (15-350 keV),
at ~0 sec after the trigger.

The XRT began observing the field at 00:44:43.60 UT, 125.6 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

Due to a telemetry gap, UVOT data are not available at this time.

Although XRT did not report an X-ray counterpart in the limited immediately
transmitted data, the significance of the BAT image (>10 sigma), the
shape of the BAT light curve, and the simultaneous detection by Fermi/GBM
(trigger 764210564) give us confidence that this is an astrophysical
Gamma Ray Burst.

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

R. Hamburg (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 00:42:39.02 UT on 21 March 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250321A (trigger 764210564/250321030),
which was also detected by Swift BAT (Gupta et al. 2025, GCN 39794
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 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 39 degrees.

The GBM light curve shows one main pulse with a duration (T90)
of about 6 s (50-300 keV). The time-averaged spectrum
from T0-0.5 to T0+7.2 s is best fit by 
a power law function with an exponential high-energy cutoff.
The power law index is -0.83 +/- 0.20 and the cutoff energy,
parameterized as Epeak, is 101 +/- 12 keV.

The event fluence (10-1000 keV) in this time interval is
(1.04 +/- 0.08)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-0.13 s in the 10-1000 keV band is 4.7 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

K.L. Page (U. Leicester) reports on behalf of the Swift-XRT team:

Using 1.7 ks of XRT Photon Counting mode data starting 134 s after the BAT
trigger on GRB 250321A (GCN Circ. 39794
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 ), an X-ray source was identified
at a position of RA, Dec = 295.09873, 21.04146, which is equivalent to

RA (J2000): 19h 40m 23.7s
Dec (J2000): 21d 02′ 29.3″

with an uncertainty of 4.2 arcsec.

This circular is an official product of the Swift-XRT team.

A. Kumar, S. Belkin, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, D. O'Neill, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco report on behalf of the GOTO collaboration:

The Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) observed the field of GRB 250321A, detected by Fermi/GBM (Fermi GBM Team, GCN 39793
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 ) and Swift/BAT (Gupta et al., GCN 39794
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 ). The position of the Swift/XRT counterpart (Page, GCN 39798
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 ) was covered by GOTO-North at 05:25:29 UT on 2025-03-21 (4.7 hours post-trigger). The observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. No optical counterpart is detected in the Swift/XRT localisation region (Page, GCN 39798
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 ) down to a 3-sigma L-band limit of 19.6 mag (AB).

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,
V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 250321A ( Fermi GBM team, GCN 39793
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 ) errorbox  25226 sec after notice time and 25261 sec after trigger time at 2025-03-21 07:43:40 UT, with upper limit up to  17.1 mag. The observations began at zenith distance = 82 deg. The sun  altitude  is -37.1 deg. 

The galactic latitude b = -6 deg., longitude l = 56 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2818762

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   25341 | 2025-03-21 07:43:40 |         MASTER-OAFA | (19h 30m 40.40s , +20d 25m 41.2s) |   C |   160 | 16.0 |        
   25537 | 2025-03-21 07:46:56 |         MASTER-OAFA | (19h 30m 47.85s , +20d 25m 03.0s) |   C |   160 | 16.4 |        
   25733 | 2025-03-21 07:50:11 |         MASTER-OAFA | (19h 30m 42.54s , +20d 24m 22.3s) |   C |   160 | 16.7 |        
   26355 | 2025-03-21 08:00:33 |         MASTER-OAFA | (19h 30m 43.53s , +20d 26m 11.7s) |   C |   160 | 16.4 |        
   27421 | 2025-03-21 08:19:20 |         MASTER-OAFA | (19h 30m 53.70s , +20d 27m 42.1s) |   C |    40 | 17.0 |        
   27431 | 2025-03-21 08:19:20 |         MASTER-OAFA | (19h 40m 32.05s , +20d 50m 27.7s) |   C |    60 | 16.6 |        
   27559 | 2025-03-21 08:20:37 |         MASTER-OAFA | (19h 30m 47.78s , +20d 27m 00.4s) |   C |   160 | 16.8 |        
   27569 | 2025-03-21 08:20:38 |         MASTER-OAFA | (19h 41m 30.19s , +20d 47m 56.2s) |   C |   180 | 16.7 |        
   27694 | 2025-03-21 08:23:53 |         MASTER-OAFA | (19h 30m 54.58s , +20d 27m 29.9s) |   C |    40 | 17.1 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,
V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250321A ( R. Gupta et al., GCN 39794
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 ) errorbox  27374 sec after notice time and 27401 sec after trigger time at 2025-03-21 08:19:20 UT, with upper limit up to  16.7 mag. The observations began at zenith distance = 78 deg. The sun  altitude  is -30.1 deg. 

The galactic latitude b = -1 deg., longitude l = 57 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2818736

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   27432 |         MASTER-OAFA |   C |    60 | 16.6 |        
   27569 |         MASTER-OAFA |   C |   180 | 16.7 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

S. P. R. Shilling (Lancaster U.) and R. Gupta (NASA/GSFC) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250321A
128 s after the BAT trigger (Gupta et al., GCN Circ. 39794
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 ).
No optical afterglow consistent with the XRT position
(Page et al., GCN Circ. 39798
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 ) is detected in the initial UVOT exposures.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           128          278          147         >19.9
white              128         4071          344         >20.6
v                 4282         4482          197         >18.9
b                 3667         5142          236         >20.0
u                  340         5097          232         >19.4
w1                4693         4892          197         >19.3
m2                4487         4687          197         >19.2
w2                4077         4277          197         >19.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 4.368 in the direction of the burst
(Schlegel et al. 1998).

E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

We have analysed 6.8 ks of XRT data for the Swift-BAT-detected burst
GRB 250321A, from 108 s to 39.4 ks after the  Swift-BAT trigger. The
data comprise 9 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. Using 1662 s
of PC mode data and 3 UVOT images, we find an enhanced XRT position
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue): RA, Dec = 295.09844, +21.04038 which is equivalent
to:

RA (J2000): 19h 40m 23.62s
Dec(J2000): +21d 02' 25.4"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is 72 arcsec from the Swift-BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.13 (+0.11, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+1.3, -1.0). The
best-fitting absorption column is  3.7 (+3.4, -2.2) x 10^22 cm^-2, in
excess of the Galactic value of 1.1 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 7.3 x 10^-11 (2.4 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.7 (+3.4, -2.2) x 10^22 cm^-2
Galactic foreground: 1.1 x 10^22 cm^-2
Excess significance: 2.0 sigma
Photon index:	     2.2 (+1.3, -1.0)

If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 6.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x
10^-14 (1.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01297508.

This circular is an official product of the Swift-XRT team.

R. Gupta (GSFC), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL),
T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC),
T. Sakamoto (AGU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250321A (trigger #1297508)
(Gupta, et al., GCN Circ. 39794
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 ).  The BAT ground-calculated position is
RA, Dec = 295.100, 21.055 deg which is
   RA(J2000)  =  19h 40m 23.9s
   Dec(J2000) = +21d 03' 16.4"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 24%.

The mask-weighted BAT light curve shows a sharp peak at the trigger time, followed by a rapid decline.
T90 (15-350 keV) is 6.21 +- 1.36 sec (estimated error including systematics).

The time-averaged spectrum from T-0.14 to T+7.35 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.61 +- 0.19.  The fluence in the 15-150 keV band is 6.7 +- 0.8 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T-0.16 sec in the 15-150 keV band
is 3.2 +- 0.5 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.
 
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1297508